1. Field of the Invention
The present invention relates to a method for substituting an inert gas for a gas (e.g., air including impurities) in a pellicle space surrounding a mask and a pellicle, which is provided to prevent foreign matter such as particles from adhering to a pattern surface. The method is suitably applied to an exposure apparatus in which a mask pattern is irradiated to a photosensitive substrate through a projection optical system by using ultraviolet light as exposure light while gas in the exposure apparatus is replaced with an inert gas. Also, the present invention relates to an exposure apparatus provided with an inert gas substituting apparatus for substituting an inert gas for a gas in the pellicle space.
2. Description of the Related Art
Hitherto, a scale-down (reduction) projection exposure apparatus for projecting and printing a circuit pattern drawn in the form of a mask as a scaled-down image on a substrate, over which a photoresist is coated, has been employed in a process for manufacturing semiconductor devices formed of very fine patterns, such as LSIs and ultra LSIs. Even finer patterns have been demanded with an increase in packing density of semiconductor devices, and exposure apparatuses have been improved to be adaptable for such a demand in step with development of the resist process.
The resolution provided by an exposure apparatus can be improved by setting the wavelength of exposure light to be shorter, or by setting the numerical aperture (NA) of a projection optical system to be greater.
The wavelength of exposure light has become shorter with the development of KrF excimer lasers having oscillation wavelengths ranging from 365 nm of the i-line to about 248 nm in a recent version, and of ArF excimer lasers having oscillation wavelengths of about 193 nm. A fluorine (F2) excimer laser having an oscillation wavelength of about 157 nm has also been developed.
In connection with an ArF excimer laser having an oscillation wavelength of about 193 nm and a fluorine (F2) excimer laser having an oscillation wavelength of about 157 nm, it is known that a plurality of oxygen (O2) absorption bands exist in the far ultraviolet range, particularly, in the ranges near those oscillation wavelengths.
Applications of a fluorine excimer laser, for example, to an exposure apparatus have progressed because it has a short oscillation wavelength of about 157 nm. The wavelength of 157 nm resides in a wavelength range that is generally called a vacuum ultraviolet range. The reason is that since, in such a wavelength range, oxygen molecules greatly absorb light and the atmosphere (air) is hardly transmissive to light, applications of a fluorine excimer laser are feasible only under an environment in which the air pressure is reduced to a level close to a vacuum and the oxygen concentration is reduced to a sufficiently low level. According to a reference xe2x80x9cPhotochemistry of Small Moleculesxe2x80x9d (by Hideo Okabe, A Wiley-Interscience Publication, 1978, p.178), the absorption coefficient of oxygen to light having a wavelength of 157 nm is about 190 atmxe2x88x921cmxe2x88x921. This means that when the light having a wavelength of 157 nm passes air at 1 atm with an oxygen concentration of 1%, the transmittance per 1 cm is that as given below:
T=exp(xe2x88x92190xc3x971 cmxc3x970.01 atm)=0.150
Further, ozone (O3) is generated upon oxygen absorbing the above light. Various materials produced from ozone adhere to the surfaces of optical devices and hence lower the efficiency of an optical system.
In an optical path of an exposure optical system of a projection exposure apparatus which employs, as a light source, a far ultraviolet ray emitted from an ArF excimer laser, a fluorine (F2) excimer or the like, therefore, the concentration of oxygen present in the optical path is reduced to a level lower than an order of several ppm by a method purging out the oxygen with an inert gas such as nitrogen.
Thus, in exposure apparatuses utilizing far ultraviolet rays, particularly, an ArF excimer laser beam having a wavelength of about 193 nm and a fluorine (F2) excimer laser beam having a wavelength of about 157 nm, oxygen in an optical path must be purged out to a level lower than an order of several ppm because the ArF excimer laser beam and the fluorine (F2) excimer laser beam are very easily absorbed by components of the exposure apparatus.
The above description is similarly applicable to moisture. That is, moisture (H2O) must also be removed to a level lower than an order of several ppm. For that reason, it has been conventional that air containing oxygen and moisture in the interior of an exposure apparatus, especially in a path of an ultraviolet ray, is purged out with an inert gas. Also, a load lock mechanism is provided in a section communicating the interior and the exterior of an exposure apparatus with each other. When a reticle or a wafer is carried into the interior of the exposure apparatus from the exterior, it is first placed in the load lock mechanism for being cut-off from the open atmosphere and for purging-out of impurities with an inert gas. Thereafter, the reticle or the wafer is carried into the interior of the exposure apparatus.
FIG. 14 is a schematic sectional view showing one example of a semiconductor exposure apparatus which employs a fluorine (F2) excimer laser as a light source and includes load lock mechanisms.
Referring to FIG. 14, reference numeral 1 denotes a reticle stage on which a reticle having a pattern drawn thereon is mounted, and 2 denotes a projection optical system for projecting the pattern on the reticle to a wafer. Numeral 3 denotes a wafer stage which mounts the wafer on it and is driven to rotate in directions of X, Y, Z, xcex8 and tilt. Numeral 4 denotes an illumination optical system for irradiating illumination light to the reticle, and 5 denotes a guiding optical system for introducing light from a light source to the illumination optical system 4. Numeral 6 denotes a fluorine (F2) excimer laser unit as the light source, and 7 denotes a masking blade for blocking off the exposure light so that only a pattern area on the reticle is illuminated. Numerals 8 and 9 denote housings provided respectively around the reticle stage 1 and the wafer stage 3 to enclose an axis of the exposure light. Numeral 10 denotes a He gas conditioner for adjusting atmospheres in the projection optical system 2 and the illumination optical system 4 to a predetermined He atmosphere. Numerals 11 and 12 denote N2 gas conditioners for adjusting atmospheres in the housings 8 and 9, respectively, to predetermined N2 atmospheres. Numerals 13 and 14 respectively denote a reticle load lock and a wafer load lock used when carrying the reticle and the wafer into the housings 8 and 9. Numerals 15 and 16 respectively denote a reticle hand and a wafer hand for carrying the reticle and the wafer. Numeral 17 denotes a reticle alignment mark used for adjusting the reticle position, 18 denotes a reticle storage device for storing a plurality of reticles in the housing 8, and 19 denotes a pre-alignment unit for making pre-alignment of the wafer. Further, if necessary, the entirety of the exposure apparatus is placed in an environment chamber (not shown), and the temperature in the environment chamber is controlled to be kept constant by circulating air controlled at a predetermined temperature in the environment chamber.
FIG. 15 is a schematic sectional view showing another example of a semiconductor exposure apparatus which employs a fluorine (F2) excimer laser as a light source and includes load lock mechanisms.
In the example of FIG. 15, a housing 20 covers the entirety of the exposure apparatus, and O2 and H2O present in the housing 20 are purged out with an N2 gas. Numeral 21 denotes an N2 gas conditioner for establishing an N2 gas atmosphere in the entire inner space of the housing 20. In this exposure apparatus, inner spaces of a lens barrel 2 and an illumination optical system 4 are isolated from the inner space (driving system space) of the housing 20 to form a He atmosphere therein independently. Numerals 13 and 14 respectively denote a reticle load lock and a wafer load lock used when carrying a reticle and a wafer into the housing 20.
Generally, a pattern protection device, called a pellicle, is attached to a reticle. The pellicle serves to prevent foreign matter, such as dust, from adhering to a pattern surface of the reticle, and to reduce the frequency of failure occurrence due to transfer of the foreign matter onto a wafer.
FIG. 16 is a schematic view showing a structure of the pellicle. A pellicle 24 is bonded to the pattern surface side of a reticle 23 using an adhesive or the like. The pellicle 24 comprises a support frame 25 having a size large enough to surround a reticle pattern, and a pellicle film 26 bonded to one end surface of the support frame 25 and allowing the exposure light to pass through it. If a space (referred to as a xe2x80x9cpellicle spacexe2x80x9d hereinafter) surrounding the reticle 23, the support frame 25 and the pellicle film 26 is completely enclosed, the pellicle film 26 may expand or contract due to a pressure difference and an oxygen concentration difference between the interior and the exterior of the pellicle space. To avoid such a drawback, a vent hole 27 is formed in the pellicle frame 25 so that gas is able to communicate between the interior and the exterior of the pellicle space. Also, a dust removing filter (not shown) is disposed in a vent passage to prevent foreign matter from entering the pellicle space through the vent hole 27.
FIG. 17 is a schematic view showing one example of reticle carrying paths in each of the exposure apparatuses shown in FIGS. 14 and 15.
Referring to FIG. 17, numeral 22 denotes a foreign matter inspecting apparatus for measuring the size and number of particles of foreign matter, such as dust, adhering to the reticle surface and the pellicle film surface. The reticle 23 is carried into the reticle load lock 13, which serves as an entrance of the exposure apparatus, manually or by a carrying apparatus (not shown). Since the reticle 23 and the pellicle 24 are generally bonded to each other outside the exposure apparatus, the pellicle 24 is already bonded to the reticle 23 at the time when the latter is carried into the reticle load lock 13. Then, air in the reticle load lock 13 is purged out with an inert gas. After an inert gas atmosphere comparable to that in the housing 8 has been formed, the reticle 23 is carried by the reticle hand 15 to one of the reticle stage 1, the reticle storage device 18 and the foreign matter inspecting apparatus 22.
As described above, in exposure apparatuses utilizing far ultraviolet rays, particularly, an ArF excimer laser beam and a fluorine (F2) excimer laser beam, oxygen and moisture greatly absorb light at the wavelengths of the ArF excimer laser beam and the fluorine (F2) excimer laser beam. Therefore, the concentrations of oxygen and moisture must be reduced in order to obtain satisfactory levels of transmittance and stability. Also, for the purpose of closely controlling those concentrations, a load lock mechanism is provided in a section communicating the interior and the exterior of the exposure apparatus with each other. When a reticle or a wafer is carried into the interior of the exposure apparatus from the exterior, it is first placed in the load lock mechanism for being cut-off from the open atmosphere and for purging-out of impurities with an inert gas. Thereafter, the reticle or the wafer is carried into the interior of the exposure apparatus.
However, a pellicle is already bonded to the reticle which is carried into the load lock chamber, and a pellicle space is communicated with an ambient atmosphere only through a relatively small vent hole. Because of such a structure, even after an inert gas in the load lock chamber has reached a predetermined concentration, a still longer time is required until air in the pellicle space is completely replaced with the inert gas, and, therefore, the productivity deteriorates.
Furthermore, regarding a vent hole formed in a pellicle frame, Japanese Unexamined Patent Application Publication Nos. 6-27643, 9-197652, and so on, disclose inventions wherein intake and exhaust holes are provided. Even with an increase in the number and area of the holes, however, a gas replacement time through the holes is still longer than that required for the load lock chamber through which air is forcibly purged out. This is because, if an assembly of the reticle and the pellicle is simply placed in an inert gas atmosphere, a diffusion phenomenon due to a difference in inert gas concentration between the interior and the exterior of the pellicle space is a primary gas replacement mechanism. When a valve or a dust removing filter is disposed in a passage communicating with the hole, the gas replacement time is further prolonged.
Also, Japanese Patent Laid-Open No. 9-73167 discloses an invention wherein a reticle and a pellicle are bonded to each other beforehand in an inert gas atmosphere so that an inert gas is sealed in a pellicle space with an oxygen concentration of not more than 1%. As described above, however, the transmittance of light having a wavelength of 157 nm is only 15% per 1 cm in a gas under the atmospheric pressure with an oxygen concentration of 1%. Considering that the air gap between the reticle and the pellicle film is about 6 mm at present, even if a gas with an oxygen concentration of 0.1% is filled in the pellicle space, the transmittance of light having a wavelength of 157 nm through the air gap is only 89.2%. On the other hand, a total space distance of an optical path from a light source to a wafer in an exposure apparatus exceeds at least 1 m. To ensure the transmittance of not more than 80% through the space of 1 m, the oxygen concentration must be suppressed to a level not more than 10 ppm V/V, and an ideal target is not more than 1 ppm. From the standpoint of making a balance with other spaces and ensuring satisfactory transmittance over the total space distance, the oxygen concentration of at least 1 to 100 ppm is required for the pellicle space. Of course, the above description is similarly applied to the concentrations of moisture and carbon dioxide as well.
Even when an inert gas is sealed in the pellicle space with an oxygen concentration of the ppm order, the following problems have occurred. Because the interface at which the pellicle frame and the reticle are bonded to each other is not of a completely airtight structure, oxygen enters the pellicle space through minute gaps if the oxygen concentration in a space where the reticle and the pellicle are placed is higher than that in the pellicle space. It has hence been very difficult to maintain the oxygen concentration of the ppm order in the pellicle space rather than the % order. In the case of the pellicle film being formed of a fluorocarbon resin, because such a resin is permeable to oxygen, it has been more difficult to maintain the oxygen concentration of the ppm order in the pellicle space. Accordingly, there has been a possibility that the reticle is mounted on the reticle stage and the exposure operation is performed in spite of the pellicle space being in a condition where the inert gas concentration is not at a sufficient level. In such a case, since the inert gas concentration in the pellicle space gradually approaches that in the ambient atmosphere while the reticle is mounted on the reticle stage, the transmittance of exposure light through the pellicle space varies over time. As a result, the predetermined amount of exposure light is not obtained on a wafer with stability, and trouble such as a dimensional change in a transferred pattern occurs.
Moreover, when a pellicle-equipped reticle is stored in an atmospheric environment outside an exposure apparatus, a large number of water molecules adhere to the surfaces of a pellicle film, a pellicle frame, etc., in many cases. Even when a pellicle-equipped reticle is stored in an inert gas environment, there is a similar problem because it may be subjected to an external atmospheric environment while being carried into the exposure apparatus.
The amount of water molecules adhering to those surfaces greatly depends upon microscopic roughness and properties of the surfaces, particularly upon whether the surfaces are hydrophilic or hydrophobic. Further, in the case of using resin materials, some types of resins absorb moisture even though in a small amount. In particular, the pellicle film and the dust removing filter are highly likely formed of fluorocarbon resin materials. This means a possibility that a large amount of moisture adheres to the surfaces of those components and is absorbed therein.
In such a case, even when air in the pellicle space is replaced with an inert gas, it is very difficult to reduce the moisture concentration to a level of the ppm order in a short period of time because water molecules adhering to the surfaces of the components and absorbed therein are gradually dissociated into the inert gas. The moisture concentration can be reduced during the purging period by sufficiently increasing the flow rate of the inert gas supplied. However, dissociation of moisture occurs continuously even after the purging process is stopped, and, hence, the moisture concentration in the small pellicle space continues to increase gradually.
When the pattern exposure is performed using such a reticle, the transmittance of exposure light through the pellicle space varies gradually over time. As a result, the predetermined amount of exposure light is not obtained on a wafer with stability, and trouble such as a dimensional change in a transferred pattern occurs.
With the view of overcoming the above-mentioned problems in the art, it is an object of the present invention to provide a method for effectively substituting an inert gas for a gas (e.g., air including impurities) in a space substantially enclosing by a reticle and a pellicle in an exposure apparatus in which a mask pattern is irradiated to a photosensitive substrate through a projection optical system by using ultraviolet light as exposure light while gas in the exposure apparatus is replaced with an inert gas.
To achieve the above object, according to a first aspect of the present invention, an exposure apparatus comprises a support for holding (i) a pellicle-equipped reticle, the pellicle-equipped reticle including a pellicle frame having an opening, and (ii) a nozzle disposed in an opposed relation to the opening formed in the pellicle frame of the reticle held on the support, the nozzle being used to substitute an inert gas for gas in a pellicle space surrounding the reticle, the pellicle frame and a pellicle film.
According to a second aspect of the present invention, a method of manufacturing a semiconductor device comprises the steps of installing a group of manufacturing apparatuses for various processes, including the exposure apparatus set forth above, in a semiconductor device manufacturing factory, and manufacturing a semiconductor device through the various processes by employing the group of manufacturing apparatuses.
According to a third aspect of the present invention, a gas substituting apparatus comprises a support for holding (i) a pellicle-equipped reticle, the pellicle-equipped reticle including a pellicle frame having an opening, and (ii) a nozzle disposed in an opposed relation to the opening formed in the pellicle frame of the reticle held on the support, the nozzle being used to substitute an inert gas for gas in a pellicle space surrounding the reticle, the pellicle frame and a pellicle film.
According to a fourth aspect of the present invention, a gas substituting method comprises the steps of holding a pellicle-equipped reticle, a pellicle-equipped reticle, the pellicle-equipped reticle including a pellicle frame having an opening, and positioning a nozzle in an opposed relation to the opening formed in the pellicle frame, and using the nozzle to substitute an inert gas for gas in a pellicle space surrounding the reticle, the pellicle frame and a pellicle film.